The eye of the 'eyeless' shrimp Rimicaris exoculata is unusual in having no image-forming optics and a high concentration of rhodopsin1. The shrimps swarm around 350°C hydrothermal 'black smoker' vents in the Mid-Atlantic Ridge2. There is no other known source of visible light in the shrimp's environment. The spectral sensitivity of rhodopsin is well matched to typical spectra of bioluminescence of organisms found at lesser depths, but other animals detect such emissions without the unusual features of the R. exoculata eye1. These two features are most easily understood as an adaptation for the detection of extremely faint sources of light. Physical calculations presented here indicate that the shrimp could see the black-body radiation of the 350°C vents, even though these sources are practically invisible to the human eye. This would be useful to the shrimp as it feeds on sulphide-loving bacteria very near to the vents3 but must avoid the lethal 350°C vents themselves.
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